Isospora buteonis Henry 1932 was considered the probable cause of death in an American kestrel (Falco sparveius) and a possible cause of death in a golden eagle (Aquila chrysaëtos) in the state of Washington, U.S.A. I. buteonis has been reported only from the western part of North America. It resembles I. bigemina in size, in location and sporulation of oocysts beneath the intestinal epithelium, and in fragility of the oocyst wall.